1. Field of the Invention
The present invention relates to an optical switch array made from single layer electrically switchable mirror that is made from holographic cholesteric liquid crystal and is polarization independent.
2. Description of the Related Art
Today's fast growing optical telecommunication technology requires sophisticated optical switch and switch array. Currently, electro-mechanical switches are widely deployed in the optical fiber network. These switches are bulky and slow in switching speed. In addition, their maintenance cost is high. Therefore, new technologies are being sought that over perform the electro-mechanical switches. Such a high performance optical switch array should exhibit non-moving parts in a fully integrated form, fast switching speed, low insertion loss, low cross talk, low polarization dispersion loss, high manufacture capability, and low cost.
Several state-of-the-art technologies have been or are being developed that offer some unique performance aspects. The first and most promising technology is invented by Agilent as disclosed by, J. E. Fouquet, “Compact optical cross-connect switch based on total internal reflection in a fluid-containing planar lightwave circuit”, OFC'2000, Baltimore, Md., Mar. 5-10, 2000, which is a fully integrated optical switch array consisting of an optical wave-guide in which a special liquid is filled. At the bottom of each intersection of two wave-guides, there is a “micro-thermal boiler”. If a switch action is desired at the spot, the “boiler” heats the liquid to the boiling point so that air bubble is created. The generated air bubble behaves like a mirror that reflects light beam into the desired wave-guide channel. However, this device consumes significant electric power and its long-term stability as well as environment stability remains questionable. The second technology is based on micro-mirror which is fabricated via micro-electrical mechanical system (MEMS) technology as disclosed by, JDS-U, Exhibition in NFOEC'2000, Denver, Colo., Aug. 27-Aug. 31, 2000 and Nortel, Exhibition in NFOEC'2000, Denver, Colo., Aug. 27-Aug. 31, 2000. This switch technology features very low insertion loss, low polarization dispersion loss and fast switching speed. However, it is difficult to accurately align the optical beams being switched between the input and output ports. In addition, its long-term lifetime remains questionable because of the mechanical motion of the mirror. The third technology is based on thermal-electric technology that switches light via thermal induced index change as disclosed by, Mitsuhiro makilara, Fusao Shimokawa, and Kazumasa Kaneko, “Strictly Non-Blocking N×N Thermo-Capillarity Optical Matrix Switch using Silica-based Waveguide”, OFC'2000, Baltimore, Md., Mar. 5-10, 2000. Again, this technology is slow and consumes high power and its thermal stability remains a question. The forth technology is based on liquid crystal phase shifter/rotator as disclosed by, Corning, Exhibition in NFOEC'2000, Denver, Colo., Aug. 27-31, 2000 and SpectraSwitch, Exhibition in NFOEC'2000, Denver, Colo., Aug. 27-Aug. 31, 2000 and Chorum, Exhibition in NFOEC'2000, Denver, Colo., Aug. 27-Aug. 31, 2000. This technology switches light without involving moving parts and is relatively fast. FIG. 1 shows the schematic diagram of the switch which is copied from the SpectraSwitch website.
In order to accomplish one switch action of a light beam, numerous optical components have to be used. The light has to pass a passive polarizing beam splitter to be split into two beams with orthogonal polarization states. Each beam has to pass an active liquid crystal phase retarder/rotator so that their polarization is modulated. Next, each beam has to be reflected by a mirror. Finally, the two beams pass another passive beam splitter where they are combined to become one single beam again. Modulation of the liquid crystal phase retarder/rotator determines the output direction of the final beam from the second beam splitter. Due to the fact that six optical elements (two beam splitters, two LC modulators and two mirrors) are involved in accomplishing one switching action, this switch exhibits a significant insertion loss. In addition, the switch is bulky and difficult for manufacturing. Furthermore, the optical modulation by the liquid crystal phase shifter or rotator is not flat within the operation spectral region (i.e., from 1,240 nm to 1,640 nm). Significant optical cross talk is expected at the wavelengths outside the liquid crystal bandwidth where the liquid crystal modulation is not sufficient. Besides, this technology has the same beam alignment problem as in MEMs technology. Finally, since this technology intrinsically depends on polarization of the light, its polarization dispersion loss is significant.